Ideally, granite is cut under controlled circumstances in a well-equipped shop. But there are times where that just isn't feasible or even possible. Sometimes you find a great deal on a granite top that was ordered but not picked up, or you are remodeling and want to fit an existing top somewhere else. Using your circular saw and a tile blade, you can cut granite anywhere you need to.
Place the granite on a pair of sawhorses, making sure that both sides of the cut will be supported. Two pairs of sawhorses aren't a bad idea, and for extra precaution you can place several 2-by-4s across the sawhorses and then place the granite on top of them.
Cover the cut line with painter's tape and mark the line on that, using a Sharpie or other magic marker. Depending on its pattern, granite may make some lines hard to see, so the tape will provide for a clean line and possibly reduce chipping.
Cut along the line, just like if you were cutting wood. Remember to go slowly and have a helper wet the blade as you cut to keep the heat down. Too much heat can warp or prematurely dull the blade.